Glass-ceramic substrate for a magnetic information storage medium
This invention relates to a glass-ceramic substrate for a magnetic information storage medium and, more particularly, to a glass-ceramic substrate for a magnetic information storage medium such as a magnetic disk made of a glass-ceramic having super flatness of a surface of the substrate suitable for use in the near contact recording system or the contact recording system. The invention relates also to a magnetic information storage medium using this glass-ceramic substrate. In this specification, the term xe2x80x9cmagnetic information storage mediumxe2x80x9d means a magnetic information storage medium in the form of a disk and includes fixed type hard disks, removable type hard disks and card type hard disks used respectively for so-called xe2x80x9chard disksxe2x80x9d for personal computers and storage of information in a network and other information storage medium in the form of a disk which can be used for storage of data in, e.g., digital video cameras and digital cameras.
Recent development of personal computers for multi-media purposes and digital video cameras and digital cameras which requires handling of a large amount of data such as moving pictures and voice has increased a demand for a magnetic information storage device of a higher recording capacity. As a result, for increasing the recording density, it is necessary in a magnetic information storage medium to increase its bit and track density and reduce the size of a bit cell. In conformity with the reduction in the size of the bit cell, a magnetic head performs its operation in closer proximity to the surface of a disk. Thus, as the magnetic head performs its operation in a near-contact state or contact state against the disk surface, it becomes important that a substrate has a super flat surface.
As described above, the magnetic head is operated in a near contact state or a contact state and the surface characteristic of the magnetic information storage medium is required to have an increased flatness compared to the conventional medium. Further, in addition to the conventional fixed type hard disks, information storage media such as removable type hard disks and card type hard disks have been proposed and put into practice and application of digital video cameras and digital cameras for various uses have been started.
Known in the art of magnetic disk substrate materials is aluminum alloy. The aluminum alloy substrate, however, has projections or spot-like projections and depressions on the substrate surface during polishing due to various defects of the material and, therefore, is not sufficient as a substrate for a high recording density storage medium in flatness and smoothness. Besides, since aluminum alloy is a soft material, deformation of the medium tends to take place. Difficulty also arises in making the magnetic information storage medium thinner. Further, damage of the medium by contact with a head is liable to occur. Thus, the aluminum alloy substrate cannot sufficiently cope with the current requirements for a high-speed recording.
As materials for overcoming the above problems of the aluminum alloy substrate, known in the art are chemically tempered glasses such as soda-lime glass (SiO2xe2x80x94CaOxe2x80x94Na2O) and alumino-silicate glass (SiO2xe2x80x94Al2O3xe2x80x94Na2O). These materials, however, have the following disadvantages: (1) Since polishing is made after the chemical tempering process, the chemically tempered layer is seriously instable in making the disk thinner. (2) For improving the contact-start-stop (CSS) characteristic, the substrate is textured to form projections or depressions on the surface. Since a mechanical or thermal (i.e., laser irradiation) processing causes cracking due to distortion in the chemically tempered layer, the chemical etching or film forming grain boundary growth method needs to be employed with resulting difficulty in producing the product in a stable manner at a low cost. (3) Since the glass contains Na2O as an essential ingredient, the glass has the problem that the film forming characteristics of the medium is deteriorated and, for preventing diffusion of Na2O, it becomes necessary to apply a barrier coating over the entire surface of the substrate. This prevents stable production of the product at a competitive cost.
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known in the art are some glass-ceramic substrates. For example, the glass-ceramics of a SiO2xe2x80x94Li2Oxe2x80x94MgOxe2x80x94P2O5 system disclosed in Japanese Patent Application Laid-open Publication No. Hei 6-329440 containing lithium disilicate (Li2O.2SiO2) and xcex1-quartz (xcex1-SiO2) as main crystal phases is an excellent material as a material textured over the entire surface in which, by controlling the grain diameter of globular crystal grains of xcex1-quartz, the conventional mechanical texturing or chemical texturing can be omitted and the surface roughness after polishing (Ra) can be controlled within a range from 15 xc3x85 to 50 xc3x85. This prior art glass-ceramic, however, cannot sufficiently cope with the requirement for the low glide height necessitated by the rapidly increasing recording density which requires an extremely small surface roughness (Ra) of 3 xc3x85 to 9 xc3x85, Further, no discussion or suggestion has been made in the patent about the landing zone which will be described later in this specification. Japanese Patent Application Laid-open Publication No. Hei 7-169048 discloses a photo-sensitive glass-ceramic of a SiO2xe2x80x94Li2O system containing Au and Ag which are photo-sensitive metals characterized by forming a data zone and a landing zone on the surface of the substrate for the magnetic disk. The predominant crystal phase of this glass-ceramic consists of lithium silicate (Li2O.SiO2) and/or lithium disilicate (Li2O.2SiO2). Lithium silicate however generally has a poor chemical durability which causes a problem in putting this glass-ceramic to a practical use. Further, in forming the landing zone, a part of the substrate (i.e., landing zone) is crystallized and is subjected to chemical etching by the HF 6% solution. It however increases thermal and mechanical instability to afford an uncrystallized portion and a crystallized portion to the disk substrate. Besides, in performing the chemical etching by the HF solution, control of concentration of the HF etching is difficult due to evaporation of the HF solution with the result that the glass-ceramic is not suitable for a large scale production.
Japanese Patent Application Laid-open Publication No. Hei 9-35234 discloses a magnetic disk substrate made of a glass-ceramic of a SiO2xe2x80x94Al2O3xe2x80x94Li2O system having predominant crystal phases of lithium disilicate (Li2O. 2SiO2) and xcex2-spodumene (Li2O.Al2O3.4SiO2). This glass-ceramic, however, contains, as its predominant crystal phase, xcex2-spodumene which has a negative thermal expansion characteristic which causes the substrate to have a negative thermal expansion characteristic and, in this glass-ceramic, growth of SiO2 crystals such as xcex1-quartz (xcex1-SiO2) and xcex1-cristobalite (xcex1-SiO2) which have a positive thermal expansion characteristic and thereby cause the substrate to have a positive thermal expansion characteristic is extremely restricted. The arithmetic mean surface roughness of this glass-ceramic after polishing is defined as 20 xc3x85 or below but the arithmetic mean surface roughness disclosed in examples is a rough one of 12 xc3x85-17 xc3x85 which fails to reach the above described desired surface roughness and, therefore, this glass-ceramic cannot cope sufficiently with the requirement for the low glide height of a magnetic head. Besides, the material which grows a crystal having a negative thermal expansion characteristic as a predominant crystal phase apparently adversely affects the substrate in difference in the coefficient of thermal expansion with respect to component parts of a drive device. Further, since this glass-ceramic requires a high temperature of 820xc2x0 C. to 920xc2x0 C. for crystallization which prevents a large scale production of the product at a competitive cost.
International Publication W097/01164 which includes the above described Japanese Patent Application Laid-open Publication No. Hei 9-35234 discloses a glass-ceramic for a magnetic disk in which temperature for crystallization is reduced (680xc2x0 C.-770xc2x0C.). A sufficient improvement however cannot be achieved in this substrate. Besides, crystals grown in all examples disclosed are xcex2-eucryptite (Li2O.Al2O3.2SiO2) which has a negative thermal expansion characteristic and this adversely affects difference in the coefficient of thermal expansion with respect to component parts of a drive device. Further, as a feature of these publications, they do not substantially comprise MgO.
It is, therefore, an object of the invention to eliminate the above described disadvantages of the prior art and provide a glass-ceramic substrate for a magnetic information storage medium having an excellent flat surface characteristic capable of coping with recording at a low glide height or contact recording of a magnetic head necessitated by increase in the recording capacity of a magnetic information storage medium.
It is another object of the invention to provide a magnetic information storage medium having a film of a magnetic medium formed on this glass-ceramic substrate.
Accumulated studies and experiments made by the inventors of the present invention for achieving the above described objects of the invention have resulted in the finding, which has led to the present invention, that a glass-ceramic which is obtained by heat treating within a limited range a SiO2xe2x80x94Li2Oxe2x80x94K2Oxe2x80x94MgOxe2x80x94ZnOxe2x80x94P2O5xe2x80x94Al2O3xe2x80x94ZrO2 system glass contains, as its predominant crystal phase or phases, at least one crystal phase selected from the group consisting of lithium disilicate (Li2O.2SiO2), quartz (SiO2), quartz solid solution (SiO2 solid solution), cristobalite (SiO2) and cristobalite solid solution (SiO2 solid solution), has crystal grains of the predominant crystal phase or phases which are fine and substantially of a globular shape, and has an excellent super-flat surface characteristic after polishing.
For achieving the above described object of the invention, there is provided a glass-ceramic substrate for a magnetic information storage medium comprising, as a predominant phase or phases, at least one crystal phase selected from the group consisting of lithium disilicate (Li2O.2SiO2, having an interplaner spacing of maximum peak intensity measured by an X-ray diffraction analysis within a range from 3.57 xc3x85 and 3.62 xc3x85), quartz (SiO2, having an interplaner spacing of maximum peak intensity measured by an X-ray diffraction analysis within a range from 3.33 xc3x85 to 3.41 xc3x85), quartz solid solution (SiO2 solid solution, having an interplaner spacing of maximum peak intensity measured by an X-ray diffraction analysis within a range from 3.33 xc3x85 to 3.41 xc3x85), cristobalite (SiO2, having an interplaner spacing of maximum peak intensity measured by an X-ray diffraction analysis within a range from 4.04 xc3x85 to 4.14 xc3x85) and cristobalite solid solution (SiO2 solid solution, having an interplaner spacing of maximum peak intensity measured by an X-ray diffraction analysis within a range from 4.04 xc3x85 to 4.14 xc3x85), has a coefficient of thermal expansion within a range from +62xc3x9710xe2x88x927/xc2x0 C. to +130xc3x9710xe2x88x927/xc2x0 C. in a temperature range from xe2x88x9250xc2x0 C. to +70xc2x0 C., has Young""s modulus within a range from 80 GPa to 150 GPa, has Vickers hardness within a range from 4.5 GPa to 15.0 GPa, has a specific gravity within a range from 2.2 to 2.8, has a surface roughness (Ra) after polishing within a range from 3 xc3x85 to 9 xc3x85 and contains Al2O3 in the amount of 2% to less than 10%.
In one aspect of the invention, the glass-ceramic substrate is substantially free of Na2O and PbO.
In another aspect of the invention, there is provided a glass-ceramic substrate wherein crystal grain diameter of lithium disilicate is within a range from 0.05 xcexcm to 0.30 xcexcm, crystal grain diameter of quartz and quartz solid solution is within a range from 0.10 xcexcm to 1.00 xcexcm and crystal grain diameter of cristobalite and cristobalite solid solution is within a range from 0.10 xcexcm and 0.501 xcexcm.
In another aspect of the invention, the glass-ceramic substrate comprises in weight percent:
In another aspect of the invention, the glass-ceramic substrate is obtained by heat treating a base glass for nucleation under a temperature within a range from 450xc2x0 C. to 550xc2x0 C. for one to twelve hours and further heat treating the glass for crystallization under a temperature within a range from 680xc2x0 C. to 800xc2x0 C. for one to twelve hours and thereafter polishing the glass-ceramic to a surface roughness (Ra) within a range from 3 xc3x85 to 9 xc3x85.
In another aspect of the invention, there is provided a magnetic information storage medium provided by forming a magnetic film and, if necessary, other layers including an undercoat layer, a protective layer and a lubricating layer, on the above described glass-ceramic substrate.